linux-kernel - Re: [PATCH] firmware: Do not use WARN_ON(!spin_is

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Message-ID: <20140813233158.GB10808@thunk.org>
Date:	Wed, 13 Aug 2014 19:31:58 -0400
From:	Theodore Ts'o <tytso@....edu>
To:	Matt Fleming <matt@...sole-pimps.org>
Cc:	Guenter Roeck <linux@...ck-us.net>,
	Matt Fleming <matt.fleming@...el.com>,
	linux-efi@...r.kernel.org, linux-kernel@...r.kernel.org,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH] firmware: Do not use WARN_ON(!spin_is_locked())

On Wed, Aug 13, 2014 at 04:02:01PM +0100, Matt Fleming wrote:
> On Wed, 13 Aug, at 07:37:37AM, Guenter Roeck wrote:
> > 
> > WARN_ON and WARN_ON_SMP are unconditional. lockdep_assert_held()
> > is only active if lockdep debugging is enabled. Not knowing the code,
> > nor the reason why the unconditional method was chosen, I prefer
> > to refrain from functional changes and limit myself to bug fixes.
>  
> As the author of that code, I feel confident telling you that the
> unconditional method was used because the author is a boob. The code
> isn't so important that we need to unconditionally check the locks, and
> indeed it's possible to run into all sorts of problems when you don't
> use the standard lock-checking functions - the non-SMP crash being a
> good example.

If you want to actually force a BUG_ON if the spinlock is not taken,
even for non-debug kernels, you can use assert_spin_locked().  This
translates to a BUG_ON(!raw_spin_is_locked(x)) on SMP kernels, and a
no-op on UP kernels.

If you're confident in your testing that any problems would be
discovered before you push your patches to linus (and you actually use
lockdep in your testing :-), then lockdep_assert_held() doesn't add
any overhead if !lockdep, and so it might be a better choice for you.

							- Ted
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